Method and apparatus for trending a physiological cardiac parameter

ABSTRACT

The present invention relates to an implantable cardioverter-defibrillator or pacemaker whose standard circuitry is used to trend a physiological cardiac parameter using intra-cardiac impedance measurements. The trend information may be used to predict the onset of a sudden cardiac death (SCD) event. By being able to predict the onset of an SCD event, patients and their physicians may be forewarned of a life-threatening event allowing them to respond accordingly. The trend information may also be used to predict the efficacy of cardiac-related medications, monitor progress of congestive heart failure, detect the occurrence of myocardial infarction, or simply track changes in sympathetic tone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/470,926, filed on Sep. 7, 2006, now issued as U.S. Pat. No.7,826,891, which is a continuation of U.S. application Ser. No.10/603,876, filed on Jun. 25, 2003, now issued as U.S. Pat. No.7,171,258, which are incorporated herein by reference, and the benefitof priority of each of which are claimed hereby.

FIELD OF INVENTION

The present system relates generally to implantablecardioverter-defibrillators and pacemakers and particularly, but not byway of limitation, to such systems being used to trend a physiologicalparameter using intra-cardiac impedance measurements.

BACKGROUND OF THE INVENTION

The heart is generally divided into four chambers, the left and rightventricles and the left and right atria. Blood passes from the rightatrium into the right ventricle via the tricuspid valve. The atrialchambers and the ventricular chambers undergo a cardiac cycle consistingof one complete sequence of contraction and relaxation of the chambersof the heart. The term systole describes the contraction phase of thecardiac cycle during which the ventricular muscle cells contract to pumpblood through the circulatory system. The term diastole describes therelaxation phase during which the ventricular muscle cells relax,causing blood from the atrial chamber to fill the ventricular chamber.After completion of the period of diastolic filling, the systolic phaseof a new cardiac cycle is initiated.

Through the cardiac cycle, the heart is able to pump blood throughoutthe circulatory system. Effective pumping of the heart depends upon fivebasic requirements. First, the contractions of cardiac muscle must occurat regular intervals and be synchronized. Second, the valves separatingthe chambers of the heart must fully open as blood passes through thechambers. Third, the valves must not leak. Fourth, the contraction ofthe cardiac muscle must be forceful. Fifth, the ventricles must filladequately during diastole.

When functioning properly, the human heart maintains its own intrinsicrhythm based on physiologically-generated electrical impulses. However,when contractions of the heart are not occurring at regular intervals,or are unsynchronized, the heart is said to be arrhythmic. During anarrhythmia, the heart's ability to effectively and efficiently pumpblood is compromised. Many different types of arrhythmias have beenidentified. Arrhythmias can occur in either the atria or the ventricles.Arrhythmias may be the result of such conditions as myocardialinfarction, cardiomyopathy or carditis.

Ventricular fibrillation is an arrhythmia that occurs in the ventriclesof the heart. In ventricular fibrillation, various areas of theventricle contract asynchronously. During ventricular fibrillation theheart fails to pump blood. If not corrected, the failure to pump bloodand thereby maintain the circulation can have fatal consequences.

Ventricular tachycardia is an arrhythmia that occurs in the ventricularchambers of the heart. Ventricular tachycardias are typified byventricular rates between 120-250 beats per minute and are caused byelectrical or mechanical disturbances within the ventricles of theheart. During ventricular tachycardia, the diastolic filling time isreduced and the ventricular contractions are less synchronized andtherefore less effective than normal. If not treated quickly, aventricular tachycardia could develop into a life-threateningventricular fibrillation.

Supraventricular tachycardias occur in the atria. Examples of theseinclude atrial tachycardias, atrial flutter and atrial fibrillation.During certain supraventricular tachycardias, aberrant cardiac signalsfrom the atria drive the ventricles at a very rapid rate.

Sudden cardiac death (SCD) may be a consequence of cardiac rhythmabnormalities occurring in the ventricles or the atria such asventricular fibrillation, ventricular tachycardia or one of thesupraventricular tachycardias. Sudden cardiac death fatally afflictsabout 300,000 Americans each year.

Patients with chronic heart disease can receive implantable cardiacdevices such as pacemakers, implantable cardioverter-defibrillators andHF cardiac resynchronization therapy devices. Implantablecardioverter-defibrillators (ICDs) are used as conventional treatmentfor patients whose arrhythmic conditions cannot be controlled bymedication. These devices provide large shocks to the heart in anattempt to revive a patient from a cardiac rhythm abnormality that mayresult in an SCD occurrence. At the present there are no firm predictorsfor SCD within these devices.

SUMMARY OF THE INVENTION

This document discusses an implantable cardioverter-defibrillator orpacemaker whose standard circuitry is used to trend a physiologicalcardiac parameter using intra-cardiac impedance measurements. The trendinformation may be used to predict the onset of an SCD event. By beingable to predict the onset of an SCD event, patients and their physiciansmay be forewarned of a life-threatening event allowing them to respondaccordingly. The trend information may also be used to predict theefficacy of cardiac-related medications, monitor progress of congestiveheart failure, detect the occurrence of myocardial infarction, or simplytrack changes in sympathetic tone.

In one embodiment of the present invention, a method of predictingsudden cardiac death includes the steps of determining intra-cardiacimpedance, deriving a physiologic cardiac parameter from the determinedimpedance, trending the parameter over spaced time intervals, andpredicting the onset of a sudden cardiac death episode.

In another embodiment, a system for predicting sudden cardiac deathepisode includes a device that measures intra-cardiac impedance, aderivation module that derives a physiological cardiac parameter fromthe measured impedance, and a module that trends the derived parameterover spaced time intervals to create trend data. The system may alsoinclude an analyzing module that analyzes the trend data to predict asudden cardiac death episode.

In a further embodiment, a method of trending a cardiac parameterincludes the steps of measuring an intra-cardiac impedance, determininga physiologic parameter using the intra-cardiac impedance, and trendingthe cardiac parameter over time.

In a yet further embodiment, a device for trending a physiologicalcardiac parameter includes an impedance module that measuresintra-cardiac impedance, a parameter module that calculates cardiacparameter values using the measured impedance, and a trending modulethat generates trend data using cardiac parameter values.

These and various other features, as well as advantages, whichcharacterize the present invention, will be apparent from a reading ofthe following detailed description and a review of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsdescribe substantially similar components throughout the several views.Like numerals having different letter suffixes represent differentinstances of substantially similar components. The drawings illustrategenerally, by way of example, but not by way of limitation, variousembodiments discussed in the present document.

FIG. 1 is a schematic/block diagram illustrating generally, among otherthings, one embodiment of portions of an impedance sensor for trending aphysiological cardiac parameter and an environment in which it is used.

FIG. 2 is a schematic/block diagram illustrating generally, among otherthings, one embodiment of portions of an impedance sensor for trending aphysiological cardiac parameter.

FIG. 3 is a schematic/block diagram illustrating generally, among otherthings, one embodiment of further portions of the measuring module ofthe impedance sensor of FIG. 2.

FIG. 4 is a schematic/block diagram illustrating generally, among otherthings, an embodiment of further portions of the parameter module of theimpedance sensor of FIG. 2.

FIG. 5 is a schematic/block diagram illustrating generally, among otherthings, an embodiment of further portions of the trending module of theimpedance sensor of FIG. 2.

FIG. 6 is a schematic/block diagram illustrating generally, among otherthings, an embodiment of further portions of the analyzing module of theimpedance sensor of FIG. 2.

FIG. 7 is a schematic/block diagram illustrating generally, among otherthings, another embodiment of portions of the impedance sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments or examples. These embodimentsmay be combined, other embodiments may be utilized, and structural,logical and electrical changes may be made without departing from thespirit and scope of the present invention. The following detaileddescription is therefore, not to be taken in a limiting sense, and thescope of the present invention is defined by the appended claims andtheir equivalents.

The present system and methods are described with respect to implantablecardiac rhythm management (CRM) devices, such as pacemakers,cardioverter defibrillators (ICDs), pacer/defibrillators, andmulti-chamber and/or multi-site (in a single chamber or multiplechambers) cardiac resynchronization therapy (CRT) devices that utilizestandard pacing and defibrillating leads. The software directingoperation of such devices may be modified in a way to utilizeintra-cardiac impedance measurements collected by the device to generatea physiological cardiac parameter. The device may also be programmed totrend the generated parameter over time. The trend information mayrepresent changes in sympathetic activity of cardiac tissue and therebybe used to track certain physiologic indicators such as, for example,the prediction of a sudden cardiac death (SCD) event, the efficacy ofcardiac-related medications being taken by the patient, the detection ofa myocardial infarction, or the progress of congestive heart failure ina patient. For example, one trend may show a slow decrease in overallsympathetic activity over time, while another trend may show a sharpdrop in sympathetic activity that is sustained for a given period oftime, while yet another trend may show spikes of sympathetic activity atcertain times during each day that may be related to how the heart isreacting during specific activities. Because certain trends may indicatea specific physiological indicator (as listed above), the system of thepresent invention may be configured to identify the occurrence ofcertain physiological indicators from trend information. Suchphysiologic parameters may be referred to as “predeterminedphysiological indicators” to the extent that the system may beconfigured to identify and track one or more specific indicators basedon the trend information.

Sympathetic activity refers to the level of activation of the autonomicnervous system, specifically the sympathetic nerves that regulatecardiac muscle contraction. Increased sympathetic activity (or tone) isan important contributor to the generation of spontaneouslife-threatening arrhythmias and SCD. Changes in sympathetic activityduring specific patient activities (such as exercise or sleep) over timemay provide important information for the patient and their physician.

There are several physiological cardiac parameters that may be generatedfrom intra-cardiac impedance measurements that provide insight intosympathetic activity by inferring their effects on cardiaccontractility. Three exemplary parameters are stroke volume, ejectionfraction, and pre-ejection period (PEP). “Stroke volume” refers to thevolume of blood pumped from a ventricle of the heart in one beat.“Ejection fraction” refers to the ratio of the volume of blood the heartempties during systole to the volume of blood in the heart at the end ofdiastole expressed as a percentage. “Pre-ejection period” measures thelatency between the onset of electromechanical systole, and the onset ofleft-ventricular ejection.

In one example, it is known that PEP shortens when sympathetic activityis increased. This shortened parameter may be measured via intra-cardiacimpedance. Therefore, should a patient experience a myocardialinfarction (MI), or have already experienced a MI, electrical remodelingwill occur in the heart. This remodeling may manifest itself as anincreased average sympathetic activity (detected by the shorted PEPvalues over some time interval), and eventually a life-threateningarrhythmia and possibly even sudden cardiac death.

The following is a detailed description of various systems and methodsof generating and trending physiological cardiac parameters based onintra-cardiac impedance that are used to track certain physiologicalindicators. FIG. 1 is a schematic/block diagram illustrating generallyone embodiment of portions of a system 100 of the present invention andan environment in which it is used. In this embodiment, system 100includes, among other things, an CRM device 105, which is coupled byleads 110, 112, 137 to heart 114. Heart 114 includes four chambers:right atrium 116, right ventricle 118, left atrium 120 and leftventricle 122. Heart 114 also includes a coronary sinus 124, a vesselthat extends from right atrium 116 toward the left ventricular freewall, and which, for the purpose of this document, is considered toinclude the great cardiac vein and/or tributary vessels.

Lead 110 may include an electrode associated with right atrium 116, suchas a tip electrode 126 and/or ring electrode 128. The electrode is“associated” with the particular heart chamber by inserting it into thatheart chamber, by inserting it into a portion of the heart's vasculaturethat is close to that heart chamber, by epicardially placing theelectrode outside that heart chamber, or by any other techniques ofconfiguring and situating an electrode for sensing signals and/orproviding therapy with respect to the heart chamber.

Lead 112, which is introduced into coronary sinus 124 and/or the greatcardiac vein or one of its tributaries, includes one or a plurality ofelectrodes associated with left ventricle 122, such as tip electrode 130and/or ring electrode 132. Lead 137 includes one or a plurality ofelectrodes associated with the right ventricle, such as tip electrode138 and/or ring electrode 140.

Device 105 may also include other electrodes, such as housing electrode134 and/or header electrode 136, which are useful for, among otherthings, unipolar sensing of heart signals or unipolar delivery ofcontraction-evoking stimulations in conjunction with one or more of theelectrodes 126, 128, 130, 132, 138, 140 associated with heart 115.Electrodes 134 and 136 may be referred to in the art as “can”electrodes, such that electrodes 126, 128, 130, 132, 138, 140 positionedin the heart may be compared to or communicate with the “can”electrodes. Alternatively, bipolar sensing and/or therapy may be usedbetween electrodes 126 and 128, between electrodes 130 and 132, betweenelectrodes 138 and 140, or between any one of the electrodes 126, 128,130, 132, 138, 140 and another closely situated electrode. In practice,any combination of unipolar and bipolar electrodes positioned within theheart may be used, in addition to combining the electrodes positionedwithin the heart with “can” electrodes to obtain the necessary impedancemeasures.

Device 105 may include several features that may be represented bymodules, process steps and components as hereinafter described. Forexample, device 105 may include a measuring module 142 that is coupledto one or more of the electrodes 126-136 for sensing electricaldepolarizations and intra-cardiac impedance corresponding with heartchamber contractions. Device 105 may also include a parameter module144, a trending module 146, an analyzing module 148, and other modulesor features relevant to tracking intra-cardiac impedance and trendingderived physiologic parameters over time. For example, device 105 mayinclude a transceiver 150 for communication between device 105 and anoutside source such as, for example, an external programmer 152, anexternal storage device 154, or an external analyzing module 156.

Referring now to FIG. 2, one embodiment of an example system or device200 for trending a physiological cardiac parameter is provided. System200 may include a measuring module 210, a parameter module 230, atrending module 250, and in some cases may further include an analyzingmodule 270. Modules 210, 230, 250 and 270 are further described hereinwith reference to FIGS. 3-6. In essence, the measuring module 210 iscapable of measuring intra-cardiac impedance values in a patient, theparameter module 230 is capable of calculating or otherwise deriving aphysiologic cardiac parameter using the measured impedance values, thetrending module 250 is capable of generating trend data using thederived parameter values, and the analyzing module 270 is capable ofanalyzing trend data to track predetermined physiological indicators. Insome embodiments, analyzing module 270 is part of a device includingmeasuring, trending and parameter modules, such as the device 105 shownin FIG. 1. In other embodiments, analyzing module 270 may be an externalanalyzing module, such as module 156 illustrated in FIG. 1, thatanalyzes trend data at a separate location from the device in which themeasuring, parameter and trending modules are located. Also, in otherembodiments, system 200 may include other modules or components such asa transceiver 150, a controller (not shown), a signal generator (notshown), etc. if such components or modules are not integrated into themeasuring, parameter, trending and analyzing modules.

FIG. 3 illustrates several functions and capabilities of measuringmodule 210 as it relates to trending device 200 of the presentinvention. Measuring module 210 may be capable of performing suchfunctions as verifying a correct position of a lead within heart 212,passing current between electrodes of the lead at spaced time intervals214, measuring voltage between electrodes of the lead 216, calculatingimpedance values from the measured voltage 218, and storing impedancevalues 220.

Verifying the correct position of a lead within heart 212 may includeverifying that the lead is correctly positioned within a heart chamber,such as chambers 116-122 of FIG. 1 (leads 126 and 128), or within avessel of the heart, such as vessel 124 shown in FIG. 1 (leads 130 and132). Verification of the correct position of the lead 212 may not be arequired function for the measuring module as the position of the leadmay be assumed to be correct when an operator of device 200 activatesthe device to begin measuring. In some cases, however, verification ofthat the lead is correctly positioned in the heart may be part of thesensing capabilities of measuring module 210.

Passing current to electrodes of the lead at spaced time intervals 214may include passing current to one or more electrodes of a lead withinthe heart, or to an electrode positioned within the heart and to aseparate electrode position external the heart (step 215 in FIG. 3),such as, for example, electrodes 134, 136 shown in FIG. 1. The currentmay be passed to the electrodes of the lead at a constant rate or atspaced time intervals. The frequency in which current is passed toelectrodes of the lead may coincide with the voltage measurements beingtaken between the electrodes of the lead 216. The voltage measurementsmay also be taken between the lead electrode and the external electrode217. Preferably, current is provided to the electrodes so that voltagemeasurements can be taken at any desired time or time interval. Forexample, voltage measurements could be taken only during what wouldtypically be when the patient is sleeping, when the patient isexercising, or any number of combinations of time periods throughout agiven day, week, etc.

The measured voltage is then used for calculating impedance values 218.The calculated impedance values may be sent directly to the parametermodule 230 shown in FIG. 4, stored within device 200, or may betransferred to an outside source for storage. Storing impedance values220 may include storing the impedance values into an array or a likeformat that reflects variables related to the voltage and impedancevalues.

The parameter module 230 may be capable of performing such functions ascollecting impedance values 232, averaging impedance values over settime intervals 234, calculating parameter values using calculatedimpedance values 236, storing calculated parameter values 238, andtransferring calculated parameter values 240 to, for example, anadvanced patient management system 242 or to another outside source 244.

Collecting impedance values may include accessing the stored impedancevalues, for example, from a stored array of impedance values. Theimpedance values may be averaged over set time intervals prior to beingused to calculate parameter values, or may be directly calculated intoparameter values. Averaging impedance values over set time intervals 234may include averaging the impedance values on, for example, a dailybasis, a weekly basis, or other desired set time interval. Thecalculated parameter values may be stored within device 200 for futureprocessing by device 200, or for future transfer of the parameter valuesto an outside source. Calculated parameter values may also be directlytransferred to a patient management system or to an outside source thatmay, in other embodiments, perform the trending and analyzing functionsof modules 250 and 270.

The trending module 250, shown in FIG. 5, may be capable of performingseveral functions. For example, trending module 250 may collectparameter values 252, trend collected parameter values over set timeintervals 254, compare trends at different times 256, transfer data to apatient management system 258, transfer data to an outside source 260,average parameter values over set time intervals 262, and trend averageparameter values over set time intervals 264.

Collecting parameter values 252 may include collecting all parametervalues stored by the parameter module 230, or collecting only certainparameter values at certain time intervals. Trending collected parametervalues over a set time interval 254 may coincide with which parametervalues are collected. Trending collected parameter values may includedetermining changes in parameter values over certain time intervals,such as, for example, changes in an average parameter value for eachhour during a 24-hour period, for each day during a 7-day week, for eachweek during a given month, or for each month over the course of a year,etc. A “trend” may be generally defined as a pattern over a period oftime, such as, for example, a net increase over time, a gradual,incremental increase over time, a steady value over time, etc. Comparingtrends at different times 256 may not be required in all embodiments oftrending module 250.

As stated above, averaging parameter values over set time intervals 256may be used for trending over set time intervals 264. Thus, eitherspecific parameter values or average parameter values may be compared toobtain trend data. Trend data may be transferred to an advanced patientmanagement system 258 or to another outside source 260 that may beassociated with device 200.

The trend data output by trending module 250 may be analyzed in severaldifferent ways. For example, trend data may be analyzed by analyzingmodule 270 that is part of device 200. In other embodiments, anindividual, or some type of analyzing system or module, such as externalanalyzing module 156 in FIG. 1, that is independent of device 200, mayperform analysis of trend data.

Analyzing module 270 may be capable of performing several functions suchas those shown in FIG. 6. For example, analyzing module 270 may collecttrend data 272, compare trend data 274, detect differences in trend data276, and transfer trend data to a patient management system 278.Analyzing module 270 may also track changes in sympathetic activity 280,monitor effects of drug regimens 282, monitor progress of congestiveheart failure 284, detect occurrence of myocardial infarction 286,predict sudden cardiac death episode 288, store results 290, andtransfer results to an outside source 292. The functions of collectingtrend data 272, comparing trend data 274 and detecting differences intrend data 276 may involve further analysis and processing of trendinformation generated by trending module 250, the results of which maybe transferred, for example, to an advanced patient management system278 or another outside source 292. The trend data that is collected,compared, and detected may be used to track certain physiologicalindicators, such as indicators 280-288.

Trend data analyzed by analyzing module 270 may be generally used totrack or monitor sympathetic activity (tone) 280. Changes in sympatheticactivity, inferred from trend data may be useful diagnostic informationfor physicians. For example, the trend data may be used to monitor theeffects of a drug or neural stimulation regimen being given a patient toalter sympathetic activity. The trend data may also be used to monitorthe progress of congestive heart failure in a patient. Monitoring trenddata related to intracardiac impedance could be used instead of R-Rinterval frequency spectrum (a conventional approach) or to augment suchfrequency-based sympathetic tone measurements.

Trend data may also be useful for detecting the occurrence of myocardialinfarction 286. This type of detection is possible because a myocardialinfarction typically triggers electrical remodeling which leads toincreased cardiac sympathetic nerve density. Thus, detecting theoccurrence of a myocardial infarction may be important because researchhas indicated that as many as one out of every three myocardialinfarctions are considered to be unnoticed by the patient. In addition,myocardial infarction is usually an eventual precursor to sudden cardiacdeath episode (SCD).

A further use of trend data may be in predicting SCD. Changes insympathetic activity, as may be inferred from certain types of trends insuch physiological parameters as described above, may indicate the onsetof an SCD.

Early recognition by a patient or the patient's physician of increasesof sympathetic activity over time (as indicated by trend data) mayprovide an opportunity for earlier treatment for the patient.

In some embodiments, the analyzing module of system 200 may be able tostore results within device 200 for future transmission to an outsidesource, or may immediately transfer results to an advanced patientmanagement system. Some advanced patient management systems may includean alarm or similar indicator that would alert the patient or thepatient's physician if, for example, a certain threshold value is met.Other patient management systems may be configured to connect to acommunications system, such as, for example, a telecommunicationssystem, the Internet via a hard landline or wireless network system, orsatellite system to automatically send patient data at spaced timeintervals or continuously send data in real time.

One example of a method of trending physiologic parameters is shown inFIG. 7. Method 300 may include the steps of measuring intracardiacimpedance 310, deriving physiologic cardiac parameters 330, trendingderived physiological parameters 350, and analyzing trend data to trackpredetermined physiological indicators 370. Each of steps 310, 330, 350and 370 may include steps or functions that coincide with thosefunctions described with reference to modules 210, 230, 250 and 270,respectively, and to systems 100 and 200 generally.

The functions performed by the system and method discussed above may beperformed by a single unitary device, such as an implantable cardiacrhythm management device. The instructions for performing the steps ofthe method and the functions related to the device discussed above maybe stored on a computer readable medium having computer executableinstructions. The present invention may also include a computer datasignal embodied in a carrier wave readable by a computing system andencoding a computer program of instructions for executing a computerprogram of instructions for executing a computer program performing themethod steps and system functions discussed above.

In some instances, various cardiac rhythm management (CRM) devices thatare currently sold and marketed may be modified in order to practice thepresent invention. For example, if a given CRM device includes hardwarecapable of performing necessary intracardiac impedance measurements, thesoftware of the system may be modified or augmented for software thatperforms the impedance measuring, parameter deriving, trending andanalyzing functions required by the present invention.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments may be used in combination with each other. Many otherembodiments will be apparent to those of skill in the art upon reviewingthe above description. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fillscope of equivalents to which such claims are entitled. In the appendedclaims, the terms “including” and “in which” are used as theplain-English equivalents of the respective terms “comprising” and“wherein.”

1. A system for monitoring congestive heart failure, the systemcomprising: a measuring device configured to measure, using a lead,intra-cardiac impedance of a patient when the patient is typicallysleeping; a derivation module configured to: derive a physiologiccardiac parameter from the measured impedance, the derived parameterincluding at least one of stroke volume, ejection fraction, andpre-ejection period; and derive sympathetic activity from thephysiologic cardiac parameter; a trending module configured to trend thederived sympathetic activity over spaced time intervals to createtrended sympathetic activity data; and an analyzing module configured toanalyze the trended sympathetic activity data to monitor congestiveheart failure status from the analysis.
 2. The system of claim 1,wherein the analyzing module is configured to: compare the trendedsympathetic activity data; and detect a difference between the trendedsympathetic activity data.
 3. The system of claim 1, comprising areporting module configured to report the trended sympathetic activitydata to an outside source.
 4. The system of claim 1, wherein thederivation module and the trending module are packaged with animplantable device.
 5. The system of claim 1, comprising an externaldevice configured to store the trended sympathetic activity data.
 6. Thesystem of claim 1, comprising a separately located device, wherein thesystem is configured to download the trended sympathetic activity datato the separately located device.
 7. The system of claim 6, wherein theseparately located device includes an external patient managementsystem.
 8. The system of claim 1, wherein the measuring device isconfigured to measure intra-cardiac impedance using an implantablecardiac rhythm management device.
 9. The system of claim 1, wherein themeasuring device is coupled to at least two electrodes configured tomeasure a voltage resulting from a current applied between the at leasttwo electrodes, wherein the voltage is used to determine theintra-cardiac impedance.
 10. A device for trending sympathetic activity,the device comprising: an impedance module configured to measure, usinga lead, an intra-cardiac impedance of a patient at spaced time intervalswhen the patient is typically sleeping; a derivation module configuredto: derive a physiologic cardiac parameter from the measured impedance,the derived parameter including at least one of stroke volume, ejectionfraction, and pre-ejection period; and derive the sympathetic activityusing the physiologic cardiac parameter; a trending module configured togenerate trended sympathetic activity data using the derived sympatheticactivity; and an analyzing module configured to analyze the trendedsympathetic activity data to monitor congestive heart failure statusfrom the analysis.
 11. The device of claim 10, comprising a reportingmodule configured to report the trended sympathetic activity data to anoutside source.
 12. The device of claim 10, wherein the impedance moduleis in communication with at least two electrodes, the at least twoelectrodes being configured to measure a voltage resulting from acurrent applied between the at least two electrodes, wherein the voltageis used to determine the intra-cardiac impedance.
 13. The device ofclaim 10, wherein the device is configured to download the trendedsympathetic activity data to an external storage device.
 14. Adevice-readable medium comprising instructions that, when performed by apreviously-implanted medical device, cause the device to perform actscomprising: measuring, using a lead, an intra-cardiac impedance of apatient at spaced time intervals when the patient is typically sleeping;deriving a physiologic cardiac parameter from the measured impedance,the derived parameter including at least one of stroke volume, ejectionfraction, and pre-ejection period; deriving sympathetic activity usingthe physiologic cardiac parameter; generating trended sympatheticactivity data using the derived sympathetic activity; analyzing thetrended sympathetic activity data; and monitoring congestive heartfailure status from the analysis.
 15. The device-readable medium ofclaim 14, wherein analyzing the trended sympathetic activity dataincludes: comparing the trended sympathetic activity data; and detectinga difference between the trended sympathetic activity data.
 16. Thedevice-readable medium of claim 14, comprising instructions that, whenperformed by the previously-implanted medical device, cause the deviceto perform acts comprising reporting the trended sympathetic activitydata to an outside source.
 17. The device-readable medium of claim 14,comprising instructions that, when performed by the previously-implantedmedical device, cause the device to perform acts comprising storing thetrended sympathetic activity data on an external device.